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The diurnal temperature range in the CMIP5 models
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
2015 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 44, no 1-2, 405-421 p.Article in journal (Refereed) Published
Abstract [en]

This paper analyzes the diurnal temperature range (DTR) over land in simulations of the recent past and in future projections by 20 models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5). The annually averaged DTR is evaluated for the present-day climate using two gridded datasets (HadGHCND and CRU). The DTR varies substantially between different CMIP5 models, particularly in the subtropics, and is generally underestimated. In future projections of the high emission scenario RCP8.5, the models disagree on both the sign and the magnitude of the change in DTR. Still, a majority of the models project a globally averaged reduction in the DTR, with an increase over Europe, a decrease over the Sahara desert and a substantial decrease in DTR at high latitudes in winter. The general DTR reduction is partly linked to the enhancement of the downwelling clear sky longwave radiation due to greenhouse gases. At high latitudes in winter, the decrease in DTR seems to be enforced by an increase in cloudiness, but in most other regions counteracted by decreases in cloud fraction. Changes in the hydrological cycle and in the clear sky shortwave radiation also impact the DTR. The DTR integrates many processes and neither the model differences in the DTR nor in the change in DTR can be attributed to a single parameter. Which variables that impact the model discrepancies vary both regionally and seasonally. However, clouds seem to matter in most regions and seasons and the evaporative fraction is important in summer.

Place, publisher, year, edition, pages
2015. Vol. 44, no 1-2, 405-421 p.
Keyword [en]
CMIP5, Diurnal temperature range, Maximum temperature, Minimum temperature, HadGHCND, Global climate models
National Category
Meteorology and Atmospheric Sciences
URN: urn:nbn:se:su:diva-103076DOI: 10.1007/s00382-014-2144-2ISI: 000347710400018OAI: diva2:715235
Available from: 2014-05-02 Created: 2014-05-02 Last updated: 2015-02-16Bibliographically approved
In thesis
1. The Representation of Atmospheric Boundary Layer Processes in Global Climate Models
Open this publication in new window or tab >>The Representation of Atmospheric Boundary Layer Processes in Global Climate Models
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The atmospheric boundary layer is the lowest part of the atmosphere, which is in direct contact with the surface. It is here, in this turbulent layer, that the exchange of heat, moisture and momentum between the surface and the atmosphere takes place. This thesis examines how well the boundary layer is described in global climate models with particular focus on the representation of the diurnal cycle. Two versions of the Community Atmosphere Model (CAM) that employ different turbulence parameterizations are evaluated in the same model framework. It is found that both overestimate the amplitude of the diurnal cycles of near-surface variables compared to observations from flux tower sites. The 10-m wind is much lower in CAM5 than in CAM4 due to the Turbulent Mountain Stress (TMS) parameterization of subgrid orography in CAM5. Additionally, the diurnal temperature range (DTR) is studied in a large set of models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The model discrepancies are large both in simulations of the present day and in projections of the future. A correlative approach is used to assess which parameters are important for the model differences in DTR. No single parameter is found to be responsible, but clouds play an important role in all seasons and so do the evaporative fraction in summer. The diurnal cycles of these CMIP5 models are also evaluated against flux tower observations. The diurnal cycle of temperature is well captured, while most variables show a large inter-model spread. A subset of the models are analyzed deeper regarding their vertical boundary layer structure for a flux site in Oklahoma. A substantial warm summer bias is revealed in the models. Finally, the impact of TMS and changes in the vertical diffusion in CAM5 is studied. It is found that, although the inclusion of TMS leads to an improved large-scale circulation, the wind turning becomes too strong, which adds to the overestimation of the ageostrophic flow in the boundary layer. Instead, increasing the diffusivity in stable conditions tends to both degrade the large-scale circulation and cause an underestimated wind turning in the boundary layer.

Place, publisher, year, edition, pages
Department of Meteorology, Stockholm University, 2014. 45 p.
The atmospheric boundary layer, Global climate models, Diurnal cycle, Flux towers, Diurnal temperature range
National Category
Natural Sciences Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
urn:nbn:se:su:diva-103053 (URN)978-91-7447-918-8 (ISBN)
Public defence
2014-06-05, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2014-05-14 Created: 2014-04-29 Last updated: 2014-05-05Bibliographically approved

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